1.1.1 - Categorical & Quantitative Variables

Variables can be classified as categorical or quantitative. Categorical variables are those that provide groupings that may have no logical order, or a logical order with inconsistent differences between groups (e.g., the difference between 1st place and 2 second place in a race is not equivalent to the difference between 3rd place and 4th place). Quantitative variables have numerical values with consistent intervals. 

Categorical variable
Names or labels (i.e., categories) with no logical order or with a logical order but inconsistent differences between groups (e.g., rankings), also known as qualitative.
Quantitative variable
Numerical values with magnitudes that can be placed in a meaningful order with consistent intervals, also known as numerical.

Example: Weight Section

A team of medical researchers weigh participants in kilograms. Weight in kilograms is a quantitative variable because it takes on numerical values with meaningful magnitudes and equal intervals.

Example: Favorite Ice Cream Flavor Section

A teacher conducts a poll in her class. She asks her students if they would prefer chocolate, vanilla, or strawberry ice cream at their class party. Preferred ice cream flavor is a categorical variable because the different flavors are categories with no meaningful order of magnitudes. 

Example: Birth Location Section

A survey asks “On which continent were you born?” This is a categorical variable because the different continents represent categories without a meaningful order of magnitudes.

Example: Children per Household Section

A census asks every household in a city how many children under the age of 18 reside there. Number of children in a household is a quantitative variable because it has a numerical value with a meaningful order and equal intervals.

Example: Highway Mile Markers Section

When a car breaks down on the highway, the emergency dispatcher may ask for the nearest mile marker. Highway mile marker value is a quantitative variable because it is numeric with a meaningful order of magnitudes. 

Example: Running Distance Section

A runner records the distance he runs each day in miles. Distance in miles is a quantitative variable because it takes on numerical values with meaningful magnitudes. 

Example: Highest Level of Education Section

A census asks residents for the highest level of education they have obtained: less than high school, high school, 2-year degree, 4-year degree, master's degree, doctoral/professional degree. This is a categorical variable. While there is a meaningful order of educational attainment, the differences between each category are not consistent. For example, the difference between high school and 2-year degree is not the same as the difference between a master's degree and a doctoral/professional degree.